1. Field of the Invention
The present invention relates to a method of and an apparatus for generating a normalizing curve which is used in an image processing apparatus which processes an image such as a process scanner.
2. Description of the Prior Art
In a process color scanner and the like, after an image of a color original is subjected to color separation to thereby obtain density signals for the respective color components, the density signals are converted into halftone dot signals with which recording of a reproduced image is performed. If there is a hue deviation in the original, color separation conditions are changed at the discretion of an operator so that the gray balance will be maintained in the reproduced image.
In a conventional method of automatically altering color separation conditions as that disclosed in U.S. Pat. No. 4,984,071 filed by the inventor of the present invention, a reference density point is determined for each color component based on density values of the brightest portion and the darkest portion of the original (i.e., highlight and shadow density values), a gradation conversion curve (i.e., normalizing curve) is established so that these density values have predetermined signal levels, and different density distributions of image data are normalized for the respective images. This makes it possible that peaks in the distributions of the density values of a bright portion and a dark portion are corrected and an image entirely having a tone of a certain color (hue deviation) is corrected.
However, in such a correction method as above (hereinafter "first conventional method"), if there is a hue deviation only in a shadow portion due to a fact that the maximum value which can be expressed by a color film, i.e., the original, is different among the respective color components, it is difficult to perform appropriate correction. That is, since the first conventional method uniformly performs correction over a wide density range, if correction to remove the hue deviation in the shadow portion is performed, a low and intermediate density region will be also corrected and the gray balance in the low and intermediate density region will be lost.
To deal with this, the inventor of the present invention has proposed a normalizing curve generation method as that disclosed in U.S. Pat. No. 5,117,293 (hereinafter "second conventional method"). In the second conventional method, in addition to the highlight and the shadow density values mentioned above, a dummy shadow density value is determined in a predetermined manner so that a normalizing curve is generated which changes along a straight line which passes through a first and a second points which are determined by said highlight density value and said shadow density value on a coordinates plane and through a third point which is determined by said first point and said dummy shadow point in the low and intermediate density region.
According to the second conventional method, by properly setting the third point (i.e., dummy shadow density value), the hue deviation near the shadow density value is corrected and a normalizing curve is changed without affected by the amount of correction made in the shadow density range in the low and intermediate density region.
Still, due to characteristics of a color film, i.e., an original, if there is differences between the respective color components in terms of a ratio of change in a density with respect to a logarithmic value of the quantity of incident light in a section from the low density range to the intermediate density range, it is impossible to appropriately correct the gray balance in the intermediate density region by the second conventional method, not to mention by the first conventional method.
In other words, since correction of the hue deviation over all density regions is performed in the first conventional method while correction only in the vicinity of the shadow density value is performed in the second conventional method, it is impossible in both the first and the second conventional methods to perform appropriate correction of the gray balance only in the intermediate density region, or in the intermediate density region and in the vicinity of the shadow density value, or in the intermediate density region and in the vicinity of the highlight density value.
An object of the present invention is to solve such a problem and to offer a normalizing curve generation method which makes it possible to perform correction of the gray balance in an intermediate density region which has been heretofore impossible.